Q. Explain digital Storage Oscilloscope (DSO) with neat diagram, (b) Give the difference between analog and digital storage oscilloscope.
Principle of operation : The availability of electronic circuitry at low cost has enabled may digital features to be added to analog oscilloscopes Example of these are generation of a trigger after an elapsed time or after a count of a number of pulses digital display of the parameters, integral voltmeter and counter remote control. However the basic oscilloscope still remains analog and uses an analog storage CRT.
A digital oscilloscope digitizes the input signal, so that all subsequent signals are digital. A conventional IS used and storage occurs in electronic digital among memory.
The input signal is digitized and stored in memory in digital from in this state it is capable of being analyzed to produce a variety of different information. To view the display on the data from memory is reconstructed in analog form. Digitizing occurs by taking a sample of the input waveform at periodic intervals. In order to ensure that no information is lost, sampling theory states that the sampling rate must be at least twice as fast as the highest frequency in the input signal.
If this is not done then aliasing will result, as shown. This requirement for a high sampling rate means that the digitizer, which is an analog to digital converter, must have a fast conversion rate. This usually requires expensive flash analog to digital converter, whose resolution decreases as the sampling rate is increased. It is for this reason that the bandwidth and resolution of a digital oscilloscope is usually limited by its analog to digital converter.
One method of overcoming the need for a high performance converter is to use an analog store.
The input signals are sampled and these are stored in an analog shift register. They can then be read out at a much slower rate to the analog to digital converter and the results stored in a digital store. This method allows operation at up to 100 mega samples per second and has the advantage that low cost analog to digital converter can be used, whose resolution does not decrease as the sampling rate is changed. The disadvantage is that the oscilloscope cannot accept data during the digitizing period, so it has a blind spot. At low sweep speed operations it is usual to switch out the analog memory feeding the analog to digital converter in real time.
Many different input channels share are used with digital storage oscilloscopes However if all these channels share a common store, through a multiplexer, then the memory available to each channel is reduced. Oscilloscopes with up to 40 channels are commercially obtainable, with a storage capability of 25000 dots. Several oscilloscopes also have floppy disc storage capability to allow non volatile storage of waveforms, which can later be recalled into the oscilloscope and manipulated.
Waveform Reconstruction: Although the input signal may be sampled at greater than twice the highest signal frequency, aliasing can still result when the output is present as a series of dots corresponding to the sampled values. This is illustrated in where the user's mind connects together the dots which are physically closet to each other rather than those which are closet on the time scale.
In the illustration it is difficult to visualize the final waveform and oscilloscopes generally have the facility to interpolate between the dots if required by the user. Two techniques are used, linear interpolation and sinusoidal interpolation. In linear interpolation, shown in a straight line is used to connect the dots together. This works well on a pulsed or square waveform, but not on a sinusoidal wave, shows that sinusoidal interpolation gives a much better fit for sine waves although it is not suitable for pulses or square waves.
Another problem with the sampling technique used in digital oscilloscopes is that it can miss short term transient or 'glitches' which occur in between the sample points. To overcome this problem envelope mode oscilloscopes may be used. These have special logic circuitry which causes the sample and digitizing circuitry to run at a high speed, independent of the setting of the display time. AT each sample tee value is compared with the previous stored sample and the higher (or lower) value stored. This is continued for the screen interval, so that for that interval the highest and lowest points are always stored. For example, suppose that an oscilloscope digitizes every 2 ms, at a given sweep speed. If a 0.1 ms transient were to occur there is a high probability that a conventional digital oscilloscope would miss it. In an envelope mode oscilloscope the input would be sampled say every 200 ms but only the highest or lowest values that occur within a 2 ms window would be stored in memory. Therefore the transient would be recorded. The sample rate of the oscilloscope is controlled by the time setting of the oscilloscope, but the analog to digital converter runs much faster.